The present invention relates to a desensitizing gum lithographic plate finisher and more specifically to a desensitizing gum lithographic plate finisher which has high hydrophilization capacity and excellent low temperature storability and which does not cause any reduction in the ink receptivity of image areas of lithographic printing plates.
Upon manufacturing a lithographic printing plate, a gumming up step is performed as the final process. In the gumming up step, a protecting agent, i.e., so-called desensitizing gum is applied onto the surface of the printing plate. This is applied to the plate for the purposes of preventing adhesion of contaminants on the plate after the plate is manufactured and till the plate is set on a printing press to thereby prevent occurrence of background contamination on printed matters and for preventing the formation of defects on the plate surface during storing the plates in the heaped up state or upon hitting them against a foreign substance. The most important purpose of the gumming up step is to prevent the lowering of the hydrophilic properties of non-image areas due to oxidization encountered when the plate surface is directly exposed to air and to enhance the hydrophilic properties thereof. For this reason, the gumming up is a step indispensable for the plate making process.
As a gumming solution, there has been used, for a long time, a composition comprising an aqueous solution of gum arabic having a concentration ranging from about 15 to 30% by weight to which additives such as surfactants, pH-adjusting agents and preservatives are optionally added.
However, gum arabic is a natural substance which is produced only in limited regions in the world and its amount of harvest is greatly influenced by various factors such as the climate at the regions. Thus the stable supply thereof is not always expected. Moreover, the desensitizing ability of gum arabic is so strong that the ink receptivity of image areas is often lowered. Therefore, it is sometimes observed that many unacceptale printed matters are formed till those having satisfactory ink concentration are surely obtained during printing operations.
Under such circumstances, many attempts have been directed to the use of a variety of water-soluble polymeric compounds as desensitizing gum lithographic plate finishers capable of replacing gum arabic.
For instance, Japanese Patent Un-examined Publication (hereunder referred to as "J.P. KOKAI") No. Sho 54-97102 (B.P. 2010298) discloses dextrin, sterabic, arabogalactan, alginic acid salts, polyacrylic acids, hydroxyethyl cellulose, polyvinyl pyrrolidone, polyacrylamide, methyl cellulose, hydroxypropyl cellulose, hydroxymethyl cellulose and salts of carboxyalkyl celluloses. Moreover, Japanese Patent Publication for Opposition Purpose to as "J.P. KOKOKU") No. Sho 54-41921 discloses pullulan and pullulan derivatives; J.P. KOKAI No. Sho 58-197091 discloses polyvinyl pyrrolidone; and J.P. KOKAI No. Sho 56-133193 (U.S. Pat. No. 4,349,391) discloses polyvinyl alcohols. However, all of these polymeric compounds are inferior in the ability to desentize non-image areas to that of gum arabic.
In addition, it is proposed to use modified starches such as carboxyalkylated starches as disclosed in J.P. KOKAI Nos. Sho 62-7595(U.S. Pat. No. 4,731,119) and Sho 629995 and phosphated starches as disclosed in J.P. KOKAI Nos. Sho 62-11692 and Sho 62-11693 (U.S. Pat. No. 4,719,172), as the desensitizing gum lithographic plate finishers in order to improve the desensitizing ability of a desensitizing gum.
However, if such modified starches as carboxyalkylated and phosphated starches are used as the desensitizing gum lithographic plate finishers, it is often observed that these starches undergo aging to thus separate out from the solution containing the same when the solutions are stored at a low temperature for a long time period and hence they cannot serve as the desensitizing gum lithographic plate finishers.
These modified starches show excellent desensitizing ability, but are inferior in the desensitizing ability to gum arabic under severe conditions which are encountered, for instance, when a developer is deteriorated to cause the reduction in its dissolving out capacity or when a part of the light-sensitive layer to be dissolved out still remains on non-image areas of a developed presensitized plate (hereunder referred to as "PS plate") for use in making lithographic printing plate due to the reduction in developability of a PS plate with time.